Method for performing a handoff by a comp set

ABSTRACT

Disclosed is a method for performing a handoff in a CoMP cluster. A user equipment measures a signal to interference-plus-noise ratio (SINR) value of each of the CoMP sets constituted by the combination of a plurality of base stations in one CoMP cluster. Then, the user equipment transmits the SINR value of each of the CoMP sets to a serving base station through a pilot strength measurement message (PSMM). Subsequently, the user equipment receives, from the serving base station, a message which instructs a handoff determined through the transmitted signal to interference-plus-noise ratio (SINR) values.

This application is the National Phase of PCT/KR2010/000504 filed onJan. 28, 2010, which claims priority under 35 U.S.C. 119(e) to U.S.Provisional Application Nos. 61/148,397 filed on Jan. 30, 2009,61/159,081 filed on Mar. 10, 2009, 61/163,030 filed on Mar. 24, 2009,61/165,520 filed on Apr. 1, 2009, 61/165,906 filed on Apr. 2, 2009 andunder 35 U.S.C. 119(a) to Patent Application No. KR 10-2009-0045446filed in the Republic of Korea on May 25, 2009, all of which are herebyexpressly incorporated by reference into the present application.

TECHNICAL FIELD

The present invention relates to a method for performing handoff and,more particularly, a method for performing, at a user equipment (UE),handoff between Coordinate Multi-Point (CoMP) sets within one CoMPcluster.

BACKGROUND ART

In a Coordinate Multi-Point (CoMP) system, two or more base stations(BSs) or cells cooperatively communicate with a user equipment (UE) inorder to improve communication performance between a BS (cell or sector)and a UE located in a shadow area. A conventional CoMP scheme may bedivided into a Multiple-Input Multiple-Output (MIMO) CoMP-JointProcessing (JP) scheme using data sharing and a CoMP-CoordinatedScheduling/Beamforming (CoMP-CS/CB) scheme.

In the CoMP-JP scheme, a UE may instantaneously and simultaneouslyreceive data from BSs which belong to the same cluster and perform CoMPand combine signals received from the BSs so as to improve receptionperformance.

In contrast, in the CoMP-CS/CB scheme, a UE may instantaneously receivedata from one BS. Scheduling or beamforming is performed such thatinterference with other BSs within a cluster to which the UE belongs isminimized.

In order to perform CoMP, a CoMP cluster which is a set of BSs which canperform a CoMP operation needs to be preferentially set. A conventionalmethod of constituting a cluster includes a method of constituting andmanaging a cluster by a BS system or a BS control system and a method ofconstituting and managing a cluster by a UE.

FIG. 1 is a diagram showing the case where a BS control systemconstitutes a cluster.

Referring to FIG. 1, first, each UE measures interference betweenneighboring BSs and the UE, averages the interference during apredetermined period, and transmits the average to the BS controlsystem. The BS control system determines interference between the BSsand the UE using the transmitted information and collects BSs which areexpected to have high performance improvement if CoMP is used, therebyconstituting a cluster. The number of BSs within one cluster may bechanged and different clusters differ in the number of BSs includedtherein. In FIG. 1, one cluster includes three BSs.

FIG. 2 is a diagram showing the case where a UE constitutes a cluster.

Referring to FIG. 2, a UE 1 and a UE 2 measure interference betweenneighboring BSs and the respective UEs and average the interferenceduring a predetermined period of time. Each UE determines interferencebetween the BSs and the UE based on the average, and collects BSs whichare expected to have high performance improvement if CoMP is used,thereby directly constituting a cluster. Then, each UE exchanges theconstituted cluster information with a BS control system. In FIG. 2, itis assumed that one cluster includes three BSs, similarly to FIG. 1. Thenumber of BSs belonging to one cluster may be changed. Differentclusters may differ in the number of BSs included therein.

If only a BS control system constitutes a cluster, the BS control systemintermittently constitutes a new cluster by applying a variation in datatransfer amount of each cluster according to movement of a UE. However,if only the BS control system constitutes and manages all clusterswithin in the network, there is a limit to the degree of a performanceimprovement obtained using CoMP.

If only each of UEs (UE 1 and UE 2) constitutes a cluster, each of theUE 1 and UE 2 may transmit information regarding a cluster variationaccording to movement of each UE to a BS using resources of a specificregion. At this time, since respective resource allocation regions ofthe UEs may overlap with each other, cooperation between the UEs isnecessary. If each of the UEs constitutes a cluster, scheduling overheadseverely increases.

As a UE moves, handoff may occur. Handoff means that, when a UE movesfrom a BS (cell or sector) to another BS (cell or sector), a channelbetween the UE and the existing BS is changed to a channel between theUE and the new BS, in order to maintain communication. Handoff may bebroadly divided into a hard handoff scheme for breaking the existingchannel and then performing connection to a new BS and a soft handoffscheme for maintaining a channel between two BSs (cells or sectors)using a Code Division Multiple Access (CDMA) scheme.

In general, a UE may classify and manage cell IDs within a system, forhandoff, which will now be briefly described. An active set includescells to which a forward data channel (or a downlink data channel) isallocated, that is, cells which are currently performing communication.A candidate set includes cells which are not included in the active setbut have sufficient field strengths. A neighbor set includes peripheralcells which may sufficiently belong to a candidate set and are notifiedthrough a neighbor list message. A remaining set includes other cells.

A UE uses thresholds T_ADD, T_DROP, T_COMP and T_TDROP, forclassification. T_ADD is a reference value for changing a cell from theneighbor set to the candidate set. T_DROP is a reference point foroperating T_TDROP in the active set and T_COMP is a reference value forcomparing field strength of a signal received from the active set.T_TDROP is a timer which operates when the field strength of a signalreceived from a cell becomes equal to or less than the T_DROP value.Hereinafter, a handoff process of a conventional UE will be described.

FIG. 3 is a diagram showing handoff state among a BS 1, a BS 2 and a BS3.

Referring to FIG. 3, it is assumed that a UE 1 which receives a datachannel allocated by the BS 1 and performs data communication with theBS 1 enters a handoff area between the BS 1 and the BS 2. That is, it isassumed that the BS 1 belongs to the active set of the UE 1 and theremaining cells belong to the neighbor set of the UE 1.

If the field strength of a signal received from the BS 2 is greater thanthe T_ADD value, the UE 1 transmits information about the field strengthof a signal received from each BS through a pilot strength measurementmessage (PSMM) to the BS 1, in order to transfer the BS 2 from theneighbor set to the candidate set. The BS 1 transmits a handoffdirection message (HDM) to the UE 1 so as to include the BS2 in thecandidate set.

Thereafter, if the field strength of the signal received from the BS 2is greater than the field strength of the signal received from the cellin the active set by the T_COMP value or more, the UE 1 transmits a PSMMto the BS 1 in order to transfer the BS 2 from the candidate set to theactive set, and the BS 1 and the BS 2 transmit an HDM to the UE 1 so asto include the BS 2 in the active set. Then, the active set includes theBS 1 and the BS 2 and the UE 1 may receive data through a combination ofdata channels of the BS 1 and the BS 2.

Thereafter, if the UE 1 gradually moves toward the BS 2 and thus thefield strength of the signal received from the BS 1 becomes less thanthe T_DROP value, the T_TDROP timer begins to operate, and, if theT_TDROP timer has expired and the field strength of the signal receivedfrom the BS 1 is less than the T_DROP value, the BS 1 is transferredfrom the active set to the neighbor set and handoff from the BS 1 to theBS 2 is completed.

It is assumed that the UE 1 simultaneously receives data from the BS 1and the BS 2 and moves toward the BS 3. If the field strength of thesignal received from the BS 3 is gradually increased and becomes greaterthan the T_ADD value, the UE 1 transmits a PSMM to the BS 1 and the BS 2in order to transfer the BS 3 from the neighbor set to the candidateset. The BS 1 and the BS 2 transmit an HDM to the UE 1 so as to includethe BS 3 in the candidate set.

Thereafter, if the field strength of the signal received from the BS 3is greater than that of the active cell by the T_COMP value, the UE 1transmits a PSMM to the BS and the BS 2 in order to transfer the BS 3from the candidate set to the active set, and the BS 1 and the BS 2transmit an HDM to the UE 1 so as to include the BS 3 in the active set.Then, the active set including the BS 1, the BS 2 and the BS 3 and theUE 1 may receive data through a combination of data channels of the BS 1and the BS 2.

Thereafter, if the UE 1 gradually moves toward the BS 3, only the BS 3is included in the active set. However, when a resource allocation stateof the BS 3 is considered, it may difficult to perform handoff to the BS3. Since handoff is conventionally determined based only on the pilotstrength measurement of each BS, a problem may occur. In the relatedart, only handoff to one BS is considered.

In addition, a method of selecting an adequate CoMP scheme from amongvarious CoMP schemes according to the current state of a UE is not yetdefined.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies ina method of performing a handoff in each of CoMP set.

The objects of the present invention are not limited to the above objectand other objects which are not described herein will become apparent tothose skilled in the art from the following description.

Technical Solution

The object of the present invention can be achieved by providing amethod of performing handoff between coordinate multipoint (CoMP) sets,at a user equipment (UE), in a CoMP cluster including a plurality ofbase stations including measuring a signal to interference plus noiseratio (SINR) value of each of the CoMP sets constituted by thecombination of a plurality of base stations within the CoMP cluster, theuser equipment transmitting the SINR value of each of the CoMP sets to afirst base station, which is a serving base station, through a pilotstrength measurement message (PSMM), and the user equipment receiving,from the first base station, a message which instructs handoff based onthe transmitted SINR value.

The method may further include simultaneously receiving signals frombase stations in a CoMP active point set corresponding to a CoMP setselected from among the CoMP sets by a base station control system.

The method may further include receiving data from a CoMP active pointset including a base station in a CoMP set selected from among the CoMPsets by a base station control system.

The SINR value of each of the CoMP sets may be measured while the userequipment operates using a joint processing scheme which cansimultaneously receive signals from base stations in each of the CoMPsets.

The SINR value of each of the CoMP sets may be measured while the userequipment operates using a coordinated silencing scheme which eliminatesan interference signal from a specific base station among base stationsin each of the CoMP sets.

The CoMP cluster may be reconstituted by the user equipment within aCoMP network cluster set constituted by a base station control system.

An another aspect of the present invention, provided herein is a methodof supporting handoff between coordinate multipoint (CoMP) sets at abase station control system in a CoMP cluster including a plurality ofbase stations including a base station control system receiving, fromthe user equipment, a signal to interference plus noise ratio (SINR)value of each of the CoMP sets constituted by the combination of theplurality of base stations within the CoMP cluster, and the base stationcontrol system determining a CoMP active point set based on the resourceallocation state of each of the CoMP sets and the SINR value of each ofthe CoMP sets.

The method may further include the base station control system selectinga CoMP coordinate point set based on a resource allocation state of eachof the CoMP sets and the SINR of each of the CoMP sets.

Advantageous Effects

A method of performing handoff within a CoMP cluster according to thepresent invention has various advantages.

First, if a user equipment supports a CoMP operation, it is possible tomore efficiently perform handoff.

Second, it is possible to improve a data rate between a user equipmentand a system by constituting an optimal CoMP set.

The effects of the present invention are not limited to theabove-described effects and other effects which are not described hereinwill become apparent to those skilled in the art from the followingdescription.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a diagram showing the case where a BS control systemconstitutes a cluster;

FIG. 2 is a diagram showing the case where a UE constitutes a cluster;

FIG. 3 is a diagram showing a handoff state among a BS 1, a BS 2 and aBS 3;

FIG. 4 is a diagram showing an example of CoMP set classification;

FIG. 5 is a diagram showing an example of CoMP set classification;

FIG. 6 is a diagram showing a state in which a UE 1 performs handoffamong a BS 1, a BS 2 and a BS 3 in each CoMP set;

FIG. 7 is a diagram showing an example of CoMP classification;

FIG. 8 is a diagram showing an example of multiple CoMP schemes used ina single network;

FIG. 9 is a diagram showing an example of a process of selecting a CoMPscheme by a UE;

FIG. 10 is a diagram showing a process of, at a network entity,acquiring information necessary to select a CoMP scheme; and

FIG. 11 is a block diagram showing an example of the structure of a UEor an access UE.

BEST MODE

Hereinafter, the preferred embodiments of the present invention will bedescribed with reference to the accompanying drawings. It is to beunderstood that the detailed description which will be disclosed withreference to the accompanying drawings is intended to describe exemplaryembodiments of the present invention, and is not intended to describe aunique embodiment through which the present invention can be carriedout. Hereinafter, the detailed description includes detailed matters toprovide full understanding of the present invention. However, it will beapparent to those skilled in the art that the present invention can becarried out without the detailed matters. For example, although certainterms are used in the following description, the terms are not limitedto these terms and these terms may be replaced with other terms. Thesame reference numbers will be used throughout the drawings to refer tothe same or like parts.

Throughout the specification, when a certain portion “includes” acertain component, this indicates that other components are not excludedand may be further included unless otherwise noted.

The following techniques may be used in various communication systemsand such communication systems may provide various communicationservices such as packet data. The techniques of the communication systemmay be used in downlink or uplink. The term “Base Station” may bereplaced with the terms fixed station, Node-B, eNode-B (eNB), accesspoint, or ABS as necessary. The term “mobile station (MS)” may also bereplaced with the term user equipment (UE), subscriber station (SS),mobile subscriber station (MSS), AMS or mobile terminal as necessary.

A transmitter refers to a node for transmitting a data or voice serviceand a receiver refers to a node for receiving a data or voice service.Accordingly, in uplink transmission, a terminal becomes a transmitterand a base station becomes a receiver. Similarly, in downlinktransmission, a terminal becomes a receiver and a base station becomes atransmitter.

As the mobile terminal of the present invention, a personal digitalassistant (PDA), a personal communication service (PCS) phone, a globalsystem for mobile (GSM) phone, a wideband CDMA (WCDMA) phone, or amobile broadband system (MBS) phone may be used.

The embodiments of the present invention can be supported by thestandard documents disclosed in wireless access systems, such as anInstitute of Electrical and Electronics Engineers (IEEE) 802 system, a3^(rd) Generation Partnership Project (3GPP) system, a 3GPP Long TermEvolution (LTE) system, and a 3GPP2 system. That is, the steps orportions, which are not described in order to make the technical spiritof the present invention clear, may be supported by the above documents.In addition, all the terms disclosed in the present document may bedescribed by the above standard documents. In particular, theembodiments of the present invention may be supported by TS25, TS36series and C.S000x series which are the standard documents of the 3GPPand 3GPP2 system.

It should be noted that specific terms disclosed in the presentinvention are proposed for the convenience of description and betterunderstanding of the present invention, and the use of these specificterms may be changed to another format within the technical scope orspirit of the present invention.

In the present invention, a base station (BS) may conceptually include acell or a sector. In addition, in the present invention, a UE, a BS anda BS control system configure a radio network and are connected throughan internal interface, a backbone network, etc.

A BS control system performs a high-level function of a serving BS and acoordinated BS and controls data and control signal transfer. Such a BScontrol system may be generally referred to as a network entity. Aserving BS (cell) may be a BS for providing main services to a UE andmay perform transmission and reception of control information on acoordinate multipoint. In this sense, the serving BS may be referred toas an anchor cell.

A Coordinate Multipoint (CoMP) system can improve throughput of an MSlocated in a boundary between cells by applying improved MIMOtransmission in a multi-cell environment. If the CoMP system is applied,it is possible to reduce inter-cell interference in a multi-cellenvironment and enable an MS to receive data from multi-cell BSs. Sinceeach BS simultaneously supports one or more MSs MS 1, MS 2, . . . , andMS K using the same radio frequency resources, it is possible to improvesystem performance. The BS may perform Space Division Multiple Access(SDMA) based on channel state information between the BS and the MS.

In the CoMP system, a serving BS and one or more coordinated BSs may beconnected to a scheduler through a backbone network. The scheduler mayreceive channel information of channel states between the MSs MS 1, MS2, . . . , and MS K and the coordinated BSs measured by the BSs BS 1, BS2, . . . , BS M through the backbone network so as to operate. Forexample, the scheduler may schedule information a coordinated MIMOoperation with respect to the serving BS and one or more coordinatedBSs. That is, the scheduler may directly instruct each BS to perform thecoordinated MIMO operation.

In the CoMP system, cells based on the same BS as a specific MS maytransmit and receive information (e.g., data, channel state information(CSI), etc.) through an internal interface or an x2 interface. However,cells based on different BSs may transmit and receive informationthrough a backhaul network, etc.

A CoMP scheme refers to a scheme for increasing data throughput of an MSlocated in a shadow area and total data throughput of a network bycooperation between two or more transmission points. A transmissionpoint may be a base station of a cell, such as a relay or a router, oranother transmission antenna point. The actual positions of thetransport points may be the same BS (intra-cell site) or different BSs(inter-cell site).

If the transmission points are on an intra-cell site basis, sincebackhaul delay is significantly less than that of other arrangement ofbase stations, it is possible to further arrange a coordinated BS (cellsite). In order to adequately operate the CoMP system, cells need to begrouped into clusters. If cells are not grouped into clusters, since anetwork entity executes downlink data scheduling with respect to thewhole network, total backhaul overhead may be significantly large.Therefore, the cells need to be adequately grouped into clusters.Hereinafter, a method of constituting a CoMP cluster will be described.

Transmission points located around each MS may be divided into thefollowing 5 categories.

A CoMP Active Point Set (CAPS) is a transmission point set which servesa specific UE and actually performs CoMP transmission, and correspondsto an active set of a conventional cluster. A CoMP Coordinated Point Set(CCPS) is a transmission point set which serves a specific UE andactually performs CoMP coordinated scheduling, and corresponds to acandidate set of the conventional cluster.

A CoMP Report Point Set (CRPS) is a transmission point set in which aspecific UE reports a channel state between a transmitter and a receiverto a network through a pilot strength measurement message. A CoMPMeasurement Point Set (CMPS) is a transmission point set in which aspecific UE measures a channel state between a transmitter and areceiver, such as pilot signal strength, and corresponds to a neighborset of the conventional cluster. A CoMP Network Clustering Set (CNCS) isa transmission point set which performs data transmission scheduling andindicates a cluster set constituted by a network. The UE may constituteand manage the cluster within the network cluster set.

In a CoMP-CS/CB scheme, a cell set for determining coordinatedscheduling for controlling interference generated from other cellsincludes cells in the CCPS. Since only a serving cell actually transmitsdata, the size of the CAPS is always 1 in the CoMP-CS/CB scheme.

Conventionally, a CoMP-JP scheme is defined as a scheme for allocating,at each transmission point, a data channel and transmitting data.However, if all transmission points which perform the CoMP-JP schemetransmit data, backhaul data is significantly increased. Thus, a newCoMP-JP is preferably defined.

In the present invention, even when each transmission point does notallocate the data channel and transmit data, if data is available ineach transmission point, it may be defined that a CoMP-JP scheme isperformed. Accordingly, dynamic cell selection in which data isavailable in each transmission point and actual transmission may occurin one preferable transport point at a specific time may be defined as aCoMP-JP scheme.

Dynamic cell selection is similar to fast cell selection. If radioresources must be allocated for a handoff and handoff frequently occurs,system load may be increased. In order to overcome such a problem andprovide a continuous packet service in a mobile environment, fast cellselection is performed. A fast cell selection algorithm serves to selectthe best cell in terms of power and code space as well as radioenvironment from among cells included in the active set so as to providea service.

In the CoMP-JP scheme, multiple transmission points for transmittingdata to a UE are cells of the CAPS. The CAPS includes transmissionpoints for actually transmitting data. Accordingly, in the CoMP-JPscheme, the size of the CAPS is greater than or equal to 1. The CAPS isa subset of the CCPS and the sizes of the CAPS, the CCPS, the CRPS orthe CMPS satisfy Equation 1.CAPS≦CCPS≦CRPS≦CMPS  Equation 1

Here, it is noted that the CAPS, the CCPS, the CRPS and the CMPS aredefined for a specific UE. In contrast, the CNCS may be defined by anetwork.

FIGS. 4 and 5 are diagrams showing examples of CoMP set classification.

In FIGS. 4 and 5, each transmission point corresponds to a cell and thestrength of a signal from each cell may be proportional to a distancebetween a cell and a UE. It is assumed that cells are divided into twoCNCSs and a UE 1 in a network and a UE 2 are respectively located in theCNCSs.

In FIG. 4, it is assumed that two CNCS regions are mutually exclusive.Accordingly, data transmission scheduling may be performed within eachCNCS. In the embodiment, for the UE 1 and the UE 2, the size of the CAPSmay be 3, the size of the CCPS may be 9, the size of the CRPS may be 15,and the size of the CMPS may be 21.

Alternatively, for the UE 1 and the UE 2, the size of the CAPS may be 3,the size of the CCPS may be 12, the size of the CRPS may be 27, and thesize of the CMPS may be 48. In this case, the size of the CoMP point setis represented by a sum of the sizes of subsets of the CoMP point set.

In FIG. 5, a first CNCS region and a second CNCS region are not mutuallyexclusive and partially overlap. A UE may be located in a boundaryregion between the first CNCS region and the second CNCS region (thatis, a CNCS overlap region). In this state, even when a stronginterference signal from another CNCS is present, a CoMP operation maybe performed between the first CNCS and the second CNCS.

By allowing the CNCS overlap region, a UE which begins to enter the CNCSoverlap region may obtain various gains from the CoMP operation betweenthe first CNCS and the second CNCS. It is noted that resource schedulingcooperation in the CNCS overlap region is performed by a combination ofthe first CNCS and the second CNCS.

FIG. 6 is a diagram showing a state in which a UE 1 performs handoffamong a BS 1, a BS 2 and a BS 3 in each CoMP set.

Referring to FIG. 6, in the state in which the UE 1 performs handoffamong the BS 1, the BS 2 and the BS 3, the UE 1 may simultaneouslyreceive data from the BS 1 and the BS 2. It is assumed that the UE 1 maysimultaneously receive a signal from a maximum of two BSs. It is assumedthat the BS 1 and the BS 2 constitute a first CoMP set, the BS 2 and theBS 3 constitute a second CoMP set, and the BS 3 and the BS 1 constitutea third CoMP set.

A CAPS for a CoMP-JP scheme and a CCPS for a CoMP-CS/CB scheme may beincluded. The BS 1, the BS 2 and the BS 3 are all included in a CNCS. Itis assumed that the BS is an anchor cell for transmitting a controlsignal such as an HDM or Channel Assignment Message (CAM) to the UE 1.

If the UE 1 gradually moves toward the BS 3, the field strength of thesignal received from the BS 3 is gradually increased. If the fieldstrength of the signal received from the BS 3 becomes greater than theT_ADD value, the UE 1 may transmit a pilot strength measurement message(PSMM) to the BS 1 and the BS 2. At this time, when the UE transmits thePSMM using a conventional handoff scheme without determining whether ornot CoMP between BSs is performed, a BS control system may not havesufficient information used to determine whether or not CoMP between BSsis performed in consideration of a resource allocation state.

For example, if it is difficult to perform CoMP with the BS 1 or the BS2 given current resource allocation of the BS 3, the UE 1 may preferablymaintain the first CoMP set of the BS 1 and the BS 2 instead ofperforming handoff to the BS 3. If handoff is performed using theconventional method, the UE 1 compares the field strengths of thesignals received from the BS 1, the BS 2 and the BS 3 so as to performhandoff to the BS 3. Accordingly, there is a need for an improvedhandoff scheme for constituting an optimal CoMP set for solving suchproblem which occurs when the UE 1 performs handoff to the BS 3.Hereinafter, an improved handoff scheme will be described.

The UE may not know the CoMP scheme which will be performed between theBS 3 and the BS 1 (or the BS 2, only the BS 1 is consideredhereinafter). Accordingly, the UE 1 may assume the CoMP scheme whichwill be performed between the BS 3 and the BS 1 and transmit informationregarding the CoMP scheme to the BS 3. The UE 1 may instantaneously andsimultaneously receive data signals from the BS 1 and the BS 2 includedin the first CoMP set. When the UE 1 performs handoff while graduallymoving toward the BS 3, the UE 1 may instantaneously compare the fieldstrengths of the signals received from the BS 1 and the BS 3 included ina third CoMP set with the T_ADD value. The CoMP schemes performed by theUE 1 and the third CoMP set (BS 1 and BS 2) will be described.

1. CoMP-JP

(1) Case where BS 1 and BS 3 Perform CoMP-JP

The UE 1 may predict the field strength of the received signal after thethird CoMP set (BS 1 and BS 3) performs CoMP-JP on the assumption thatthe third CoMP set (BS 1 and BS 3) performs CoMP-JP. Prediction of thefield strength of the received signal after CoMP-JP may be changedaccording to a selected specific CoMP-JP scheme.

A value predicted when the third CoMP set (BS 1 and BS 3) transmits thesame data signal may be a sum of the field strengths of the signalsreceived from the third CoMP set (BS 1 and BS 2). The UE 1 compares thepredicted value with the T_ADD value and transmits informationindicating that the predicted value is greater than the threshold to theBS 1 through a PSMM if the predicted value is greater than thethreshold.

The PSMM may include the field strength information of the signalsreceived from the BS 1, BS 2 and BS3 and the field strength informationof the signals received from the first CoMP set (BS 1 and BS 2), thesecond CoMP set (BS 2 and BS 3) and the third CoMP set (BS 3 and BS 1).The PSMM may be replaced with reference signal received power (RSRP) orreference signal received quality (RSRQ).

In a Long Term Evolution (LTE) system, a UE may measure a channelquality state between the UE and a cell using RSRP corresponding topower of a pilot signal.

RSRP refers to a linear average of power distributed to resourceelements, to which cell-specific reference signals are allocated, withina measurement frequency bandwidth. Power of each resource element in aresource block may be determined from energy received in a valid periodof a symbol excluding a cyclic prefix (CP). RSRP may be applied to a UEin both a radio resource control_idle (RRC_idle) state and RRC_connectedstate. If a user uses receiver diversity, a reported value may be equalto a linear average of power values of all diversity branches.

An RRC_idle state refers to a state in which a UE checks systeminformation and paging information in a low power consumption state andthen sleeps. An RRC_connected state refers to a state in which a UEperforms telephone communication or data communication. In general, inthe case where a communication channel with the existing cell is brokenand a communication channel with another cell is established in anRRC_connected state, it indicates handover and, in the case where acommunication channel with the existing cell is broken and acommunication channel with another cell is established in a RRC_idlestate, it indicates cell reselection.

A UE may measure RSRP by accumulating pilot signals transmitted fromcells for a specific time within a predetermined bandwidth. A UE maytransmit field strength information using RSRP and transmit signal tointerference plus noise ratio (SINR) information using RSRQ.

The BS control system which receives the PSMM from the UE may determinewhether or not data is transmitted to the UE through a pair of the BS 1and BS 3, that is, whether or not the pair of BS 1 and BS 3 is includedin an active set of the UE 1, in consideration of resource allocation.

(2) Case where the BS 1 and BS 3 do not Perform CoMP-JP

In this case, the UE 1 performs handoff to the BS 3 using a conventionalscheme and may transmit a PSMM to the BS control system if the fieldstrength of the signal received from the BS 3 is greater than the T_ADDvalue. The PSMM includes field strength information of the signalsreceived from the BS 1, the BS 2 and the BS 3 and the BS control systemwhich receives the PSMM may determine whether or not a signal istransmitted through the BS 3, that is, whether or not the third CoMP setincluding the BS 1 and BS 3 is included in the active set of the UE 1.

As described above, the UE 1 may perform a process of calculating thefield strength information of the signals of the first to third CoMPsets and a process of additionally transmitting the calculated result tothe BS control system on the assumption that CoMP-JP is performed. Adetermination as to whether or not the pair of BS 2 and BS 3 of thesecond CoMP set is included in the active set may be made through thesame process in the above description.

2. CoMP-CS/CB

The BS 1, BS 2 and BS 3 establish one CoMP-CS/CB cluster and the UE 1may instantaneously receive a data signal from the BS 1 or BS 2. At thistime, handoff performed while the UE 1 gradually moves toward the BS 3will be described in the case where the UE 1 performs CoMP-CS/CB betweenthe BS 1 and BS 3 and the case where the UE 1 does not performCoMP-CS/CB between the BS 1 and BS 3.

(1) Case where the BS 1 and BS 3 Perform CoMP-CS/CB

The UE 1 may predict the field strength of the received signal after thethird CoMP performs CoMP-CS/CB on the assumption that the third CoMP set(BS 1 and BS 3) performs CoMP-CS/CB. Prediction of the field strength ofthe received signal after CoMP-CS/CB may be changed according to theselected CoMP-JP scheme.

A silencing CoMP-CS/CB scheme may be used. For example, in the case ofsilencing the BS 3, the predicted value of the field strength of thereceived signal may be represented by a signal to interference plusnoise ratio (SINR). On the assumption that an interference signal fromthe BS 3 is not present, the SINR may be calculated by subtractinginterference from the received signal from the BS 3.

The UE 1 compares the predicted value with the T_ADD value and transmitsinformation indicating that the predicted value is greater than thethreshold to the BS 1 through a PSMM if the predicted value is greaterthan the threshold. The PSMM may include the field strengths of thesignals received from the BS 1, BS 2 and BS3 and the field strengthinformation of the first to third CoMP sets. The BS control system whichreceives the PSMM from the UE may determine whether or not data istransmitted to the UE through the third CoMP (BS 1 and BS 3), that is,whether or not the third CoMP is included in a candidate set of the UE1, in consideration of resource allocation.

The BS control system may select a BS (that is, an active set) whichwill actually transmit a signal to the UE from the candidate set if thecandidate set is set. Therefore, in case of CoMP-CS/CB, the size of theactive set is always 1 and the BS control system may manage thecandidate set by aid of the UE.

(2) Case where BS 1 and BS 3 do not Perform CoMP-CS/CB

The UE 1 performs handoff to the BS 3 using a conventional scheme andmay transmit a PSMM to the BS control system if the field strength ofthe signal received from the BS 3 is greater than the T_ADD value. ThePSMM includes field strength information of the signals received fromthe BS 1, the BS 2 and the BS 3 and the BS control system which receivesthe PSMM may determine whether or not a signal is transmitted throughthe BS 3, that is, whether or not the third CoMP set (BS 1 and BS 3) isincluded in the candidate set of the UE 1. If the candidate set is set,the BS control system may select a BS of the active set which willactually transmit a signal to the UE 1.

Therefore, as compared to the conventional scheme, there is a need for aprocess of calculating the field strength information of the signals ofthe first to third CoMP sets and a process of transmitting thecalculated result to the BS control system on the assumption thatCoMP-CS/CB is performed.

A determination as to whether the second CoMP set (BS 2 and BS 3) isincluded in the candidate set may be made through the same process asthe above description and thus a description thereof will be omitted.

As described above, in the case where a BS, which is newly consideredupon handoff of a UE, performs CoMP-JP (or CoMP-CS/CB) with a BSbelonging to the existing CoMP set and vice versa, the field strengthinformation of a received signal of each BS is transmitted through aPSMM such that the BS control system newly constitutes a CoMP set.

Field strength information and SINR of received signal between a pair ofBSs according to each CoMP scheme may be calculated by a UE, a BS or aseparate device.

A network entity may not adequately determine which CoMP set is used byusing only RSRP or RSRQ. Therefore, it is possible to calculate andmeasure the SINR of a combination of signals received from cells and toprovide the information regarding the SINR. This information ispreferably reported to each cell again.

Table 1 shows feedback information of a UE which performs CoMP.

TABLE 1 SINR 1 SINR_CS 12 SINR 2 SINR_CS 13 SINR 3 SINR_CS 21 SINR_JP 12SINR_CS 23 SINR JP 13 SINR_CS 31 SINR JP 23 SINR_CS 32

Referring to Table 1, SINR N denotes an SINR from a BS N (N=1, 2, 3).SINR_JP NM denotes an SINR of a combination of signals from a BS N and aBS M on the assumption that the BS N and the BS M are included in aCoMP-JP set (where, N=1, 2, 3 and M=1, 2, 3). SINR_CS NM denotes an SINRof a combination of signals from a BS N and a BS M on the assumptionthat the BS N and the BS M are included in a CoMP-CS/CB set (where, N=1,2, 3 and M=1, 2, 3). For example, if a silencing scheme is applied inthe CoMP CS/CB scheme, SINR_CS 12 may represent a value in the case ofeliminating interference from the signal received from the BS 3 when aserving BS is 1.

The amount of information regarding an SINR necessary for a set forperforming CoMP-CS/CB is greater than the amount of informationregarding an SINR necessary for a set for performing CoMP-JP. This isbecause SINR_JP NM is equal to SINR_JP MN, but SINR-CS NM is not alwaysequal to SINR_CS MN. SINR_JP NM and SINR_CS NM may be approximatelycalculated using Equations 2 and 3.SINR_JPNM=SINRN+SINRM−α  Equation 2SINR_CSNM=SN/|IN−SM|−β  Equation 3

where, SN denotes the strength of a reference signal received from acell N and SM denotes the strength of a reference signal received from acell M. IN denotes an interference and noise coefficient of a UE 1. ∥ isa symbol representing an absolute value and |IN−SM| denotes an absolutevalue of a difference between the interference and noise coefficient andthe strength of the reference signal received from the cell N. Inaddition, α denotes a JP compensation coefficient and is equal to orgreater than 0. β denotes a CS compensation coefficient and is equal toor greater than 0.

The UE 1 may not know which CoMP scheme is currently being performed bythe third CoMP set (BS 1 and BS 3) or the second CoMP set (BS 2 and BS3). In this case, the UE may transmit all the feedback information shownin Table 1 to the BS 1, in order to cover all CoMP schemes.

However, if the UE 1 knows that the third CoMP set (BS 1 and BS 3)performs the CoMP-JP scheme, the UE 1 may transmit feedback informationSINR 1, SINR 2, SINR 3, SRNR_JP 12, SINR_JP 13 and SINR_JP 23 in Table 1to the BS 1.

In contrast, if the UE 1 knows that the third CoMP set (BS 1 and BS 3)performs the CoMP-CS/CB scheme, the UE 1 may transmit feedbackinformation SINR 1, SINR 2, SINR 3, SINR_CS 12, SINR_CS 13, SINR_CS 23,SINR_CS 31 and SINR_CS 32 in Table 1 to the BS 1.

In case of the CoMP-JP scheme, if the UE 1 receives the same data fromthe BS 1 and BS 3 constituting the third CoMP set, since SINR is a sumof the SINRs of the two signals, it is possible to calculate an SINRusing only reference signal reception power. In contrast, if the UE 1receives different data from the BS 1 and BS 3, SINR may not becalculated by the sum of the SINRs of the two signals.

Up to now, a process of performing, at a UE, a handoff in a state inwhich three BSs constitute a cluster is described. However, the numberof BSs constituting the cluster is not limited to 3. The above-describedprinciple may be applied to the case where three or more BSs constitutea cluster.

In summary, in the present invention, if a UE supports a CoMP operation,it is possible to more efficiently perform handoff. By constituting anoptimal CoMP set, it is possible to improve a data rate of a UE and BScontrol system.

<Adaptive Selection Method of Comp Scheme>

As described above, a CoMP scheme may be largely divided into a CoMP-JPscheme and a CoMP-CS/CB scheme. In a process of selecting an adaptiveCoMP scheme at a UE, a CoMP-CS/CB scheme may be divided into twoschemes: a CoMP beamforming scheme and a CoMP silencing scheme.

FIG. 7 is a diagram showing an example of CoMP set classification.

Referring to FIG. 7, one CoMP set may be added to the five CoMP setsdefined in FIGS. 4 and 5. The added CoMP set is a CoMP coordinatedsilencing point set and is defined to support adaptive selection of theCoMP scheme. The CoMP coordinated silencing point set is a set oftransmission points for serving a specific UE by performing CoMPcoordinated silencing.

Coordinated silencing means that a neighbor transmission point (CoMPsilencing point set; CSPS) does not transmit a signal to a target UEduring a specific time and a frequency separation in order to reduceinterference. An example of selecting a neighbor transmission point(CSPS) is shown in FIG. 5.

Referring to FIG. 7, two cells to be used as a CCPS are selected as acurrent neighbor transmission point (CSPS) and the two cells may nottransmit a data signal to the UE 1 during a predetermined time intervalin order to reduce interference.

A network scheduler may select one CoMP scheme from a predefined set ofcandidate CoMP schemes and apply the selected CoMP scheme to all UEswhich can operate within the range thereof. However, an adequate CoMPscheme preferably satisfies a minimum data rate requirement whichdiffers between UEs and multiple CoMP schemes which will be used in asingle network need to be allowed.

FIG. 8 is a diagram showing an example of multiple CoMP schemes used ina single network.

Referring to FIG. 8, a pair of BSs (BS A and BS B) may serve a UE 1using a CoMP-JP scheme and another pair of BSs (BS C and BS D) may servea UE 2 using a CoMP-JP scheme. Simultaneously, the BS serving the UE 2may serve the UE 1 using a CoMP beamforming scheme and the BS servingthe UE 1 may serve the UE 2 using a CoMP beamforming scheme.

In the present invention, on the assumption that multiple CoMP schemesare allowed, an adaptive CoMP scheme selection method including aplurality of selection steps for satisfying a minimum data raterequirement of each UE is provided.

If a CoMP scheme which will be used by a UE is selected, a minimum datarate requirement of a target UE is preferably considered. A data rate ofa UE may depend on factors such as a downlink (or forward link) channelstate and a CoMP scheme selected by a network.

For example, when a UE moves toward a boundary region of a cell having adownlink channel state inferior to that of a central region of the cell,a method for satisfying a minimum data rate requirement of 9.6 Kbps usesa CoMP scheme. For example, it is assumed that a CoMP-CS/CB scheme isselected. When a UE gradually moves toward the boundary region of thecell, a downlink channel state further deteriorates and thus a minimumdata rate requirement may not be satisfied even using the CoMP-CS/CBscheme.

In order to solve this problem, several BSs within a CCPS performingCoMP CS/CB are silenced. By silencing a neighbor BS so as to furtherreduce interference from the cell, it is possible to efficientlyincrease a data rate and SINR of a UE.

It is assumed that a network supports multiple CoMP schemes. At thistime, CoMP schemes supported by the network need to be defined first.For example, if the network supports CoMP schemes such as acoordinate-beamforming scheme, a coordinate-silencing scheme and aCoMP-JP scheme, the order of CoMP schemes may be defined as follows suchthat interference is gradually decreased and n SINR is graduallyincreased in the arrangement order of CoMP schemes.

Order of Comp Schemes={Coordinate-Beamforming Scheme,Coordinate-Silencing Scheme, CoMP-JP Scheme}

Hereinafter, a method of selecting an adaptive CoMP scheme of a UE basedon the order of CoMP schemes will be described.

If a UE does not satisfy a minimum data rate requirement, the UE mayselect a first CoMP scheme from the predefined order of CoMP schemes. Inthis case, the first CoMP scheme is a coordinate-beamforming scheme. IfSINR is further reduced while the UE performs the coordinate-beamformingscheme, the UE may not satisfy the minimum data rate requirement again.Then, the UE may select and use a next CoMP scheme in the predeterminedorder of CoMP schemes. The next CoMP scheme may be acoordinate-silencing scheme. In selection of a cell to be silenced, acell which may cause significantly high interference and increase SINRwith respect to a target UE is preferably selected.

If the SINR is further reduced while the UE performs thecoordinate-beamforming scheme and the coordinate-silencing scheme, theUE may not satisfy the minimum data rate requirement again. Then, the UEmay select and use a next CoMP scheme in the predetermined order of CoMPschemes. The next CoMP scheme may be a CoMP-JP scheme.

In selection of cells by which the CoMP-JP scheme is performed, a cellwhich can maximize an SINR of a target UE is preferably selected. If theUE cannot satisfy the minimum data rate requirement using the CoMP-JPscheme, a link is preferably dropped.

FIG. 9 is a diagram showing an example of a process of selecting a CoMPscheme at a UE.

Referring to FIG. 9, first, the order of CoMP schemes to be selected bythe UE may be set according to a predefined rule (S905). If the currentdata rate of the UE is less than a minimum data rate requirement (S910),the UE selects a first CoMP scheme according to the predefined order ofCoMP schemes (S915) and, otherwise, an operation is continuouslyperformed during a predetermined time T1 (S920).

Thereafter, the UE may select cells which can maximize SINR (S925). Theoperation can be performed using the selected CoMP scheme during thepredetermined time T1 (S930). Thereafter, the data rate of the UE iscompared with the minimum data rate requirement and the operation iscontinuously performed using the selected first CoMP scheme during thepredetermined time if the data rate of the UE is greater than theminimum data rate requirement. In contrast, if the current data rate ofthe UE is less than the minimum data rate requirement (S935), the UE mayselect a second CoMP scheme (S940).

If the second CoMP scheme is not the last CoMP scheme (S945), the UE mayselect cells which can maximize SINR (S950). Then, the UE maycontinuously perform the operation using the second CoMP scheme during apredetermined time T2 (S955). In contrast, if the second CoMP scheme isa last CoMP scheme, the UE may stop the operation using the second CoMPscheme and drop a link (S960). At this time, the link dropped by the UEmay be a downlink (which may be referred to as a forward link).

In a process of selecting a silencing cell (CSPS), if the number ofspecific cells to be silenced is extremely large, the throughput of thecells may be significantly less than those of other cells in a network.The throughputs of the cells may not be equal and thus the throughputsof the UEs may not be equal.

In order to solve this problem, the UE may select cells to be silencedusing a round robin scheme. That is, cells in the CCPS are sequentiallyselected one by one. A predetermined number of cells is selected duringa specific time.

As another method, a method of checking the total throughputs of thecells and silencing a cell having the highest total throughput may beused. Using this method, a cell which transmits the largest amount ofdata to a UE within a cell range is silenced first. If the cell issilenced, a selection indicator is increased one by one and indicatesthat this cell is selected for silencing. Accordingly, upon nextselection, a cell having the largest selection indicator value may beexcluded from selection of the cell to be silenced.

Hereinafter, a method of acquiring, at a network entity, informationnecessary to select a CoMP scheme will be described.

The network entity may directly determine whether each UE satisfies agiven minimum data rate requirement using data transmitted to each UE inthe past or acquire additional information from each UE and determinewhether each UE satisfies a given minimum data rate requirement.

FIG. 10 is a diagram showing a process of acquiring informationnecessary to select a CoMP scheme at a network entity.

Referring to FIG. 10, a UE may transmit a message or help indicatorindicating that the UE does not currently satisfy a minimum data raterequirement to a serving BS (S1010). The help indicator may include 1bit or may include channel state information required for selecting aCoMP scheme. The network entity may receive a report message of the UEfrom the serving BS (S1020).

The serving BS may be event-triggered by the report message so as totransmit a channel state information-request indicator (CSI-RI) forrequesting channel state information (CSI) to the UE (S1030). Theserving BS may be a single BS or multiple BSs.

Thereafter, the UE may transmit the CSI to the serving BS through anaperiodic control channel when receiving the CSI-RI (S1040). In the casewhere the UE periodically transmits CSI to the serving BS, resources maybe wasted by resource allocation for a large amount of channel stateinformation. Thus, the CSI is preferably aperiodically transmitted.

The CSI transmitted from the UE to the serving BS may include an SINR ofeach CoMP set and each BS and CSI. The SINR matches a data rate inone-to-one correspondence and may be a criterion representing a datarate. The serving BS may transmit the CSI received from the UE to thenetwork entity (S1050).

Thereafter, the network entity may adaptively select a CoMP schemesuiting a current situation.

As described above, although the embodiment in which three differentCoMP schemes are used is described in the present invention, a method ofselecting a CoMP scheme from among other CoMP schemes may be used.

FIG. 11 is a block diagram showing an example of the structure of a UEor an access UE.

Referring to FIG. 11, the UE includes a processor (or a digital signalprocessor (DSP)) 110, an RF module 135, a power management module 105,an antenna 140, a battery 155, a display 115, a keypad 120, a memory130, a subscriber identification module (SIM) card 125 (optional), aspeaker 145 and a microphone 150.

A user inputs instruction information such as a telephone number bypressing a button of the keypad 120 or using the microphone 150. Themicroprocessor 110 receives and processes the instruction informationand performs a suitable function such as telephone number dialing. Themicroprocessor may obtain operation data from the SIM card 125 or thememory 130 and perform the above function. The processor 110 may displaythe instruction and operation information on the display 115 forreference and convenience.

The processor 110 may send the instruction information to the RF module135 and start communication such as transmission of a RF signalincluding voice communication data. The RF module 135 includes areceiver and transmitter for receiving and transmitting the RF signal.The antenna 140 facilitates transmission and reception of the RF signal.The RF module 135 may receive the RF signal, convert the RF signal intoa baseband signal, and send the baseband signal to the processor 110.

The processed signal is, for example, converted into an audio signaloutput through the speaker 145 or readable information. The processor110 includes protocols and functions necessary to perform the variousprocesses described in the present specification.

The aforementioned embodiments are achieved by combination of structuralelements and features of the present invention in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be implemented without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment. Moreover, it will be apparent that someclaims referring to specific claims may be combined with other claimsreferring to the other claims other than the specific claims toconstitute the embodiment or add new claims by means of amendment afterthe application is filed.

The embodiments according to the present invention can be implemented byvarious means, for example, hardware, firmware, software, or acombination thereof. If the embodiment according to the presentinvention is implemented by hardware, the embodiment of the presentinvention can be implemented by one or more application specificintegrated circuits (ASICs), digital signal processors (DSPs), digitalsignal processing devices (DSPDs), programmable logic devices (PLDs),field programmable gate arrays (FPGAs), processors, controllers,microcontrollers, microprocessors, etc.

If the embodiment according to the present invention is implemented byfirmware or software, the embodiment of the present invention may beimplemented by a module, procedure, or function, which performsfunctions or operations described above. Software code may be stored ina memory unit and then driven by a processor. The memory unit may belocated inside or outside the processor to transmit and receive data toand from the processor through various well known means.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

[Industrial Applicability]

Methods of performing handoff in each CoMP set according to theembodiments of the present invention are industrially applicable.

1. A method of performing handoff between coordinate multipoint (CoMP)sets at a user equipment (UE), in a CoMP cluster including a pluralityof base stations (BSs), the method comprising: measuring a signal tointerference plus noise ratio (SINR) value of each of the CoMP setsconstituted by the combination of a plurality of base stations withinthe CoMP cluster; transmitting the SINR value of each of the CoMP setsto a first base station, which is a serving base station, through apilot strength measurement message (PSMM); and receiving, from the firstbase station, a message which instructs handoff based on the transmittedSINR value.
 2. The method of claim 1, further comprising: simultaneouslyreceiving signals from base stations in a CoMP active point setcorresponding to a CoMP set selected from among the CoMP sets by a basestation control system.
 3. The method of claim 1, further comprising:receiving data from a CoMP active point set configured one base stationin a CoMP set selected from among the CoMP sets by a base stationcontrol system.
 4. The method of claim 1, wherein the SINR value of eachof the CoMP sets is measured while the UE operates using a jointprocessing scheme which can simultaneously receive signals from basestations in each of the CoMP sets.
 5. The method of claim 4, wherein theSINR value is represented by a sum of SINR values of the receivedsignals if the UE receives the same signal from the base stations in theCoMP set.
 6. The method of claim 1, wherein the SINR value of each ofthe CoMP sets is measured while the UE operates using a coordinatedscheduling/beamforming scheme which eliminates an interference signalfrom a specific base station among base stations in each of the CoMPsets.
 7. The method of claim 1, wherein the CoMP cluster isreconstituted by the UE within a CoMP network cluster set constituted bya base station control system.
 8. A method of supporting handoff betweencoordinate multipoint (CoMP) sets at a base station control system in aCoMP cluster including a plurality of base stations, the methodcomprising: receiving, from the user equipment (UE), a signal tointerference plus noise ratio (SINR) value of each of the CoMP setsconstituted by the combination of the plurality of base stations withinthe CoMP cluster; and determining a CoMP active point set based on aresource allocation state of each of the CoMP sets and the SINR value ofeach of the CoMP sets.
 9. The method of claim 8, further comprising:selecting a CoMP coordinate point set based on the resource allocationstate of each of the CoMP sets and the SINR of each of the CoMP sets.10. The method of claim 8, wherein the selected CoMP active point setincludes one base station.